When testing cards at below -64°C the NVIDIA driver gets confused and thinks the GPU overheats which causes a loss in performance.
When started with /fixcoldbug the attached GPU-Z build will increase the temperature reading that the NVIDIA driver sees by 65°C (it does not actually increase GPU temperature). This change should result in stable benching at lower temperatures.
Please note that a warm reboot might not remove the change, powering off the system and back on will.